This invention relates to a modulator used for quadrature phase shift keying and more particularly to a digitized quadrature phase shift keying modulator whose internal circuits are all realized with digital circuits.
A known method for replacing internal circuits of a quadrature phase shift keying modulator with digital circuits uses, as a digital filter for shaping the transmitting waveform, a digital filter using a look-up table. This digital filter has a read only memory (ROM) in which impulse responses of a filter have been stored in accordance with such factors as the modulation output frequency and sampling rate, and waveforms of a desired impulse response corresponding to an input digital signal are read out of the ROM and synthesized together through digital operations.
The known method, however, utilizes the impulse response of a filter which has been stored in the ROM at a predetermined sampling rate and therefore, the output frequency of the modulator is determined by a data rate of the input signal. More particularly, when the data rate of the input signal changes, the modulation output frequency also changes and in order to obtain a modulation output signal at a fixed frequency, the contents of the ROM must be rewritten.
Conversely, when the data rate of the input signal is fixed, only a modulation output signal at a constant frequency is obtained, requiring that the contents of the ROM be rewritten if the modulation output frequency is desired to be changed for a fixed input signal.
In order that the waveforms of impulse response of I data and Q data of quadrature phase shift keying which are 90.degree. out of phase from each other can be delivered out of the digital filter for shaping at different timings from each other, it is necessary for the above known method to use separate or independent waveform shaping digital filters with a look-up table for I data and Q data.
For example, reference may be made to JP-A-59-207768 relevant to a method for realization of all the signal processings in internal circuits of the modulator with digital processings by using a digital filter with look-up table.